# Reservoir Sampling

## <img src="/files/IJKc2uLo7cU0ydnVJ6O3" alt="" data-size="line"> Reservoir Sampling

### Description <a href="#description" id="description"></a>

The Reservoir Sampling transform allows you to sample a fixed number of rows from an incoming data stream when the total number of incoming rows is not known in advance.

The transform uses uniform sampling; all incoming rows have an equal chance of being selected.

This transform is particularly useful when used in conjunction with the ARFF output transform in order to generate a suitable sized data set to be used by WEKA.

The reservoir sampling transform uses [Algorithm R](https://en.wikipedia.org/wiki/Reservoir_sampling) by Jeffery Vitter.

| Hop Engine | <sup>✓</sup> |
| ---------- | ------------ |
| Spark      | ?            |
| Flink      | ?            |
| Dataflow   | ?            |

### Options

| Option         | Description                                                                                                                                                    |
| -------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Transform name | Name of the transform this name has to be unique in a single pipeline.                                                                                         |
| Sample size    | Select how many rows to sample from an incoming stream. Setting a value of 0 will cause all rows to be sampled; setting a negative value will block all rows.  |
| Random seed    | Choose a seed for the random number generator. Repeating a pipeline with a different value for the seed will result in a different random sample being chosen. |


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